EAL

NA: Effect on Blood Pressure (2009)

Citation:

Study Design:

Class:

- Click here for explanation of classification scheme.

Quality Rating:

Research Purpose:

To examine north-south blood pressure differences in China and to assess nutritional factors that may be responsible.

Inclusion Criteria:

Aged 40 to 59 years.

Exclusion Criteria:

None specifically mentioned.

Description of Study Protocol:

Recruitment: Data from the International Study on Macronutrients and Blood Pressure (INTERMAP) on 4,680 participants from China, Japan, the UK and the USA. A population-based sample of 260 persons (one per family) was randomly selected from each target population, stratified by gender and 10-year age group.
Design: Cross-sectional Study
Intervention: Blood pressure and assessment of dietary intake.

Statistical Analysis

For descriptive statistics, means and standard deviations or percentages are given for each sample, with T-tests and chi-square tests for north-south comparisons
Based on significant north-south differences for non-dietary and dietary variables, multiple linear regression models were used to examine relations of dietary factors with north-south differences in SBP and DBP with control for non-dietary variables
Interactions between BMI and single nutrients in influencing BP were also assessed
Analyses were also repeated with exclusion of participants using antihypertensive medication.

Data Collection Summary:

Timing of Measurements

Each person attended the local INTERMAP center on four occasions: Two pairs of consecutive days, about three to six weeks apart.

Dependent Variables

Blood pressure measured twice with random zero sphygmomanometer.

Independent Variables

Daily nutrient intakes of sodium and potassium determined from four 24-hour recalls recorded by trained and certified interviewers and two timed 24-hour urine collections
Data collected by interviewer-administered questionnaire about age, education, occupation, work and leisure physical activity, smoking history, previous medical history, medication use, daily alcohol consumption.

Control Variables

Age
Gender.

Description of Actual Data Sample:

Initial N: 839 subjects; 561 northern, 278 southern
Attrition (final N): 839 subjects
Age: 40 to 59 years
Ethnicity: Chinese
Anthropometrics: There were differences between northern and southern Chinese
Location: China.

Summary of Results:

Variable	Beijing (North)	Shanxi (North)	North	Guangxi (South)
Number of subjects	272	289	561	278
SBP, mm Hg	122.4±17.8	124.9±18.9	123.7±18.4	116.3±14.1
DBP, mm Hg	76.5±9.8	74.5±10.9	75.5±10.4	68.6±8.0
BMI	23.8±3.3	23.7±3.6	23.8±3.5	21.8±2.6
Calcium, mg/1,000 kcal	153±50	124±41	138±48	176±62
Magnesium, mg/1,000 kcal	110±22	157±37	134±39	198±28
Phosphorus, mg/1,000 kcal	347±65	408±73	378±76	562±67
Vitamin A, IU/1,000 kcal	1706±1208	1405±847	1551±1048	3606±2171
Vitamin C, mg/1,000 kcal	41.6±18.1	32.2±15.7	36.8±17.5	45.0±21.9
Urinary Na, mmol/24-hour	275±90	268±85	271±88	139±57
Urinary K, mmol/24-hour	37.0±11.8	37.1±11.2	37.1±11.5	40.6±14.7
Urinary Na/K ratio	7.8±2.5	7.5±2.3	7.6±2.4	3.7±1.5

Other Findings

Average systolic/diastolic pressure levels were 7.4/6.9mm Hg higher for northern than southern participants (P<0.001)
Southern participants had lower BMI, sodium intake, sodium-potassium ratio and higher intake of calcium, magnesium, phosphorus and vitamins A and C
Among nutrients, urinary sodium-potassium ratio produced the largest reduction in SBP coefficient (-56.4%), followed by phosphorus (-49.0%)
Considered singly, with control for age and gender, several dietary variables (BMI, urinary sodium-potassium ratio, urinary sodium, dietary phosphorus and magnesium) reduced north-south blood pressure differences by over 10%
Controlled for age and gender, non-dietary variables had little effect on north-south blood pressure differences
With inclusion in regression models of multiple dietary variables (sodium, potassium, magnesium or phosphorus, BMI), north-south blood pressure differences became much smaller and statistically non-significant (systolic, -1.1mm Hg; diastolic, 1.6mm Hg).

Author Conclusion:

In conclusion, INTERMAP data from middle-aged Chinese participants from the north and south showed that southerners had much lower BP levels and generally more favorable lifestyles, especially lower BMI, sodium intake, sodium-potassium ratio and higher intake of magnesium and phosphorus (also other micronutrients)
These largely accounted for their lower BP levels
Compared with northern Chinese, the nutrient intake of southern Chinese more closely resembles the DASH-sodium combination diet, demonstrated to reduce BP levels significantly in pre-hypertensive and hypertensive individuals.

Funding Source:

Government:

NHLBI

Not-for-profit

Foundation associated with industry:

Reviewer Comments:

Population-based sample and urinary excretion and four-day food intake data.

Quality Criteria Checklist: Primary Research
Relevance Questions
	1.	Would implementing the studied intervention or procedure (if found successful) result in improved outcomes for the patients/clients/population group? (Not Applicable for some epidemiological studies)	Yes
	2.	Did the authors study an outcome (dependent variable) or topic that the patients/clients/population group would care about?	Yes
	3.	Is the focus of the intervention or procedure (independent variable) or topic of study a common issue of concern to dieteticspractice?	Yes
	4.	Is the intervention or procedure feasible? (NA for some epidemiological studies)	Yes

Validity Questions
1.	Was the research question clearly stated?		Yes
	1.1.	Was (were) the specific intervention(s) or procedure(s) [independent variable(s)] identified?	Yes
	1.2.	Was (were) the outcome(s) [dependent variable(s)] clearly indicated?	Yes
	1.3.	Were the target population and setting specified?	Yes
2.	Was the selection of study subjects/patients free from bias?		Yes
	2.1.	Were inclusion/exclusion criteria specified (e.g., risk, point in disease progression, diagnostic or prognosis criteria), and with sufficient detail and without omitting criteria critical to the study?	Yes
	2.2.	Were criteria applied equally to all study groups?	Yes
	2.3.	Were health, demographics, and other characteristics of subjects described?	Yes
	2.4.	Were the subjects/patients a representative sample of the relevant population?	Yes
3.	Were study groups comparable?		Yes
	3.1.	Was the method of assigning subjects/patients to groups described and unbiased? (Method of randomization identified if RCT)	N/A
	3.2.	Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline?	N/A
	3.3.	Were concurrent controls or comparisons used? (Concurrent preferred over historical control or comparison groups.)	N/A
	3.4.	If cohort study or cross-sectional study, were groups comparable on important confounding factors and/or were preexisting differences accounted for by using appropriate adjustments in statistical analysis?	Yes
	3.5.	If case control study, were potential confounding factors comparable for cases and controls? (If case series or trial with subjects serving as own control, this criterion is not applicable.)	Yes
	3.6.	If diagnostic test, was there an independent blind comparison with an appropriate reference standard (e.g., "gold standard")?	N/A
4.	Was method of handling withdrawals described?		N/A
	4.1.	Were follow-up methods described and the same for all groups?	N/A
	4.2.	Was the number, characteristics of withdrawals (i.e., dropouts, lost to follow up, attrition rate) and/or response rate (cross-sectional studies) described for each group? (Follow up goal for a strong study is 80%.)	N/A
	4.3.	Were all enrolled subjects/patients (in the original sample) accounted for?	N/A
	4.4.	Were reasons for withdrawals similar across groups?	N/A
	4.5.	If diagnostic test, was decision to perform reference test not dependent on results of test under study?	N/A
5.	Was blinding used to prevent introduction of bias?		N/A
	5.1.	In intervention study, were subjects, clinicians/practitioners, and investigators blinded to treatment group, as appropriate?	N/A
	5.2.	Were data collectors blinded for outcomes assessment? (If outcome is measured using an objective test, such as a lab value, this criterion is assumed to be met.)	N/A
	5.3.	In cohort study or cross-sectional study, were measurements of outcomes and risk factors blinded?	N/A
	5.4.	In case control study, was case definition explicit and case ascertainment not influenced by exposure status?	N/A
	5.5.	In diagnostic study, were test results blinded to patient history and other test results?	N/A
6.	Were intervention/therapeutic regimens/exposure factor or procedure and any comparison(s) described in detail? Were interveningfactors described?		Yes
	6.1.	In RCT or other intervention trial, were protocols described for all regimens studied?	Yes
	6.2.	In observational study, were interventions, study settings, and clinicians/provider described?	N/A
	6.3.	Was the intensity and duration of the intervention or exposure factor sufficient to produce a meaningful effect?	Yes
	6.4.	Was the amount of exposure and, if relevant, subject/patient compliance measured?	Yes
	6.5.	Were co-interventions (e.g., ancillary treatments, other therapies) described?	Yes
	6.6.	Were extra or unplanned treatments described?	Yes
	6.7.	Was the information for 6.4, 6.5, and 6.6 assessed the same way for all groups?	Yes
	6.8.	In diagnostic study, were details of test administration and replication sufficient?	N/A
7.	Were outcomes clearly defined and the measurements valid and reliable?		Yes
	7.1.	Were primary and secondary endpoints described and relevant to the question?	Yes
	7.2.	Were nutrition measures appropriate to question and outcomes of concern?	Yes
	7.3.	Was the period of follow-up long enough for important outcome(s) to occur?	Yes
	7.4.	Were the observations and measurements based on standard, valid, and reliable data collection instruments/tests/procedures?	Yes
	7.5.	Was the measurement of effect at an appropriate level of precision?	Yes
	7.6.	Were other factors accounted for (measured) that could affect outcomes?	Yes
	7.7.	Were the measurements conducted consistently across groups?	Yes
8.	Was the statistical analysis appropriate for the study design and type of outcome indicators?		Yes
	8.1.	Were statistical analyses adequately described and the results reported appropriately?	Yes
	8.2.	Were correct statistical tests used and assumptions of test not violated?	Yes
	8.3.	Were statistics reported with levels of significance and/or confidence intervals?	Yes
	8.4.	Was "intent to treat" analysis of outcomes done (and as appropriate, was there an analysis of outcomes for those maximally exposed or a dose-response analysis)?	N/A
	8.5.	Were adequate adjustments made for effects of confounding factors that might have affected the outcomes (e.g., multivariate analyses)?	Yes
	8.6.	Was clinical significance as well as statistical significance reported?	Yes
	8.7.	If negative findings, was a power calculation reported to address type 2 error?	N/A
9.	Are conclusions supported by results with biases and limitations taken into consideration?		Yes
	9.1.	Is there a discussion of findings?	Yes
	9.2.	Are biases and study limitations identified and discussed?	Yes
10.	Is bias due to study's funding or sponsorship unlikely?		Yes
	10.1.	Were sources of funding and investigators' affiliations described?	Yes
	10.2.	Was the study free from apparent conflict of interest?	Yes